1. Field of the Invention
The present invention relates generally to magnetic heads for disk drives or tape drives, and relates more specifically to magnetic write heads having magnetic layer structures.
2. Description of the Related Art
Electromagnetic transducers, such as heads for disk drives or tape drives, commonly include one or more magnetically “soft” layers. The soft layers have high magnetic permeabilities and low magnetic coercivities, and can be used as good conductors of magnetic flux. Examplary magnetically soft materials include Permalloy (approximately Ni0.81Fe0.19), which can be formed in thin layers to create magnetic feature. For example, an inductive head may include a magnetic core comprising Permalloy in which adjacent conductive coils induce magnetic flux. The flux within the magnetic core is then employed to magnetize a portion (e.g., a data bit) of an adjacent media. The inductive head may also read signals from the media by magnetic induction (e.g., bringing the magnetic core near the magnetized media portion so that the flux from the media portion induces a flux in the magnetic core, the changing flux in the core then inducing an electric current in the coils). Alternatively, instead of inductively sensing media fields, magnetoresistive (MR) sensors or merged heads that include MR or giant magnetoresistive (GMR) sensors read signals by sensing a change in electrical resistance of the sensor due to the magnetic signal.
The sizes of transducer elements have decreased for many years in attempts to increase the magnetic storage density. However, smaller transducer elements are more prone to magnetic saturation, in which the elements (e.g., magnetic pole layers) are saturated by the magnetic flux, such that any additional flux is not conducted through the element. Magnetic saturation is particularly troublesome when the ends of the pole layers closest to the media (commonly termed pole tips) are saturated. Such magnetic saturation limits the amount of flux transmitted through the pole tips, thereby limiting the amount of flux available for the writing or reading of signals. Moreover, such magnetic saturation also blurs the writing or reading of signals, because the magnetic flux is more evenly dispersed over an entire pole tip instead of being focused in a corner that has relatively high flux density.
To deliver higher magnetic flux through the poles, high magnetic saturation materials (also known as high moment or high BS materials) have been used in magnetic core elements. For instance, iron is known to have a higher magnetic moment than nickel, so increasing the proportion of iron compared to nickel generally yields a higher moment alloy. Iron, however, is also more corrosive than nickel, which imposes a limit to the concentration of iron that is feasible for many applications.
It is difficult to achieve soft magnetic properties for primarily-iron NiFe alloys (e.g., alloys with an atomic concentration of iron that is greater than the atomic concentration of nickel) compared to primarily-nickel NiFe alloys (e.g., alloys with an atomic concentration of nickel that is greater than the atomic concentration of iron). The magnetic softness of a material depends on both the microstructure and the intrinsic properties of the material. The intrinsic properties include the magneto-crystalline anisotropy and the magneto-elastic energy, which depends on the magnetostriction and stress. Materials with higher magnetostriction and magnetic-crystalline anisotropy are generally magnetically-harder. For example, although FeCo has a high magnetic moment of approximately 2.45 Tesla (or 24.5 kilogauss), it also has relatively higher magnetostriction magneto-crystalline anisotropy. Therefore, FeCo normally demonstrates magnetically-hard properties.
Anderson et al., in U.S. Pat. No. 4,589,041 teach the use of high moment Ni0.45Fe0.55 for pole tips. Anderson et al. do not use Ni0.45Fe0.55 throughout the core due to problems with permeability of that material, which Anderson et al. suggest is due to relatively high magnetostriction of Ni0.45Fe0.55. As noted in U.S. Pat. No. 5,606,478 to Chen et al., the use of high moment materials has also been proposed for layers of magnetic cores located closest to a gap region separating the cores. Also noted by Chen et al. are some of the difficulties presented by these high moment materials, including challenges in forming desired elements and corrosion of the elements once formed. Chen et al. state that magnetostriction is another problem with Ni0.45Fe0.55, and disclose magnetic heads having Permalloy material layers that counteract the effects of magnetostriction. This balancing of positive and negative magnetostriction with plural NiFe alloys is also described in U.S. Pat. No. 5,874,010 to Tao et al.
Primarily-iron FeCo alloys (e.g., alloys with an atomic concentration of iron greater than the atomic concentration of cobalt) have a very high saturation magnetization but also a high magnetostriction that makes these materials unsuitable for many head applications. Due to the high magnetostriction, mechanical stresses created during slider fabrication or use can perturb the desirable magnetic domain patterns of the head. FIG. 1 is a plot of a B/H loop of an exemplary prior art FeCoN layer that was formed by sputtering deposition at room temperature. The FeCoN layer has a thickness of approximately 500 Angstroms and has a composition of approximately Fe0.66Co0.28N0.06. The applied H-field is shown in oersteds (Oe) across the horizontal axis of FIG. 1 while the magnetization (B) of the FeCoN layer is plotted in normalized units along the vertical axis of FIG. 1, with unity defined as the saturation magnetization. The FeCoN layer has a saturation magnetization (BS) of approximately 24.0 kilogauss and is magnetically isotropic, as shown by the single B/H loop. The B/H loop also indicates a relatively high coercivity of about 80 oersteds, which may be unsuitable for applications requiring soft magnetic properties.
In an article entitled “Microstructures and Soft Magnetic Properties of High Saturation Magnetization Fe-Co-N Alloy Thin Films,” Materials Research Society, Spring meeting, Section F, April 2000, N. X. Sun et al. report the formation of FeCoN films having high magnetic saturation but also having high magnetostriction and moderate coercivity. Sun et al. also report the formation of a thin film structure in which FeCoN is grown on and capped by Permalloy, to create a sandwich structure having reduced coercivity but compressive stress. The magnetostriction of this sandwich structure, while somewhat less than that of the single film of FeCoN, may still be problematic for head applications.